The country's establishment and top leaders of the ruling United Malays National Organisation (UMNO) party lashed out at the post saying many of the comments the 90-year-old leader made were seditious in nature.

"We have begun investigation a few days ago. If there is a case we will refer it to the Attorney-General," the Inspector-General said, according to the New Straits Times.

The former PM is being probed under Section 500 of the Penal Code which provides for a maximum two-year jail term and a fine.

Mahathir and his son Mukhriz, who was recently ousted as the chief minister of Kedah province, have been in the frontline of political attack of Najib, whose reign is tainted with allegations of corruption and the poor state of economy.

Mahathir has repeatedly said Najib should resign. He says the UNMO party, which has been in power since 1957, will lose the next election if it faces people under Najib's leadership.

Umno Supreme Council member Tan Sri Annuar Musa said the content of Mahathir's blog were seditious.

He said Mahathir also insulted the king when he said "the current AG was appointed by the PM through false representation made to the King".

"This claim is indeed an insult to the King, making it appear as if the King is a foolhardy person who can be easily deceived by the people," Malaysian Insider quoted him as saying.

Mahathir responded to the development saying it clearly indicated what the authorities plan to do.

"And the new publicity chief suggests I am doing this because I want to be arrested. That is as clear a warning as can be of the wish to arrest me."

Following the uproar over his blog post Mahathir sharpened his criticism of Najib saying there should be an audit of the lifestyle of leaders who stand accused in the graft probe involving 1Malaysia Berhad funds.

"Are they living within their means? If it was beyond their means, a thorough investigation must be made on all sources of income of the suspect."

Mahathir also attacked the AG saying he went beyond his brief in clearing Najib's name.

"In Malaysia the Attorney General decides if a law has been broken or not. That decision is in fact a judgement. The A.G. is both a judge and a prosecutor. This in itself is an injustice."